In view of its wide availability and ease of purification, ricin has been employed as a toxic and lethal agent by totalitarian regimes and, recently, by terrorist groups. Ricin is a member of a family of ribosome-directed toxins whose toxicity stems from the depurination of a single adenine within the "sarcin/ricin" loop of 28S ribosomal RNA (28S rRNA). The depurination of 28S rRNA results not only in the inhibition of protein translation, but also the intense and extended activation of the stress-activated protein kinases such as JNK and p38 MAPK. These kinases are thought to be central mediators of inflammatory responses that are responsible for inducing the transcription of proinflammatory cytokines and chemokines. The development of interventional remedies in cases of poisoning by ricin and other ribosome-directed toxins will depend critically on our improved understanding of the primary target tissues affected and the mechanisms that drive the proinflammatory and cytotoxic responses. When administered to cultured cells or to mice, ricin potently induces the activation of genes that encode proinflammatory cytokines and chemokines and the transcription factors that are known to drive their expression. Simultaneously, ricin activates apoptotic pathways, via engagement of apical caspases 8 and 9, which lead to cell death. Our preliminary studies support the notion that the health risks that accompany ricin intoxication stem from ricin's activation of inflammatory and apoptotic pathways. In this application we propose to employ ricin in both cultured cells and in wild-type and "knockout" mice to elucidate the initial targets of action, the cytotoxic consequences, and the cellular and molecular mechanisms that are pursuant to intoxication by ricin. In this application we propose to: 1) identify target tissues and cell types affected by ricin in a mouse model of ricin intoxication;2) identify genes whose expression is induced by ricin in specific cell and tissue types;3) determine the mechanisms of apoptosis triggered by ricin and the roles of apoptosis in the course of ricin-induced inflammation;and 4) elucidate the roles of specific inflammatory or pro-apoptotic genes in mediating the response to ricin intoxication by employing mouse "knock out" models.